The broad purpose of this proposal is to study the intrarenal mechanisms controlling sodium reabsorption from a new vantage point. In the proposed experiments we plan to utilize our recent observation that a prompt partial reduction in renal artery pressure prevents the natriuresis induced by ECF volume expansion, to attempt to uncover the peritubular factors involved in the control of both proximal and distal sodium reabsorption. The initial experiments will be designed to determine where within the nephron a reduction in renal artery pressure prevents the decrease in sodium reabsorption normally induced by volume expansion. Subsequently, we plan to use this observation to examine: 1. The role of changes in interstitial hydrostatic and oncotic pressure in the control of proximal sodium reabsorption. 2. The role of medullary hypertonicity in the control of sodium reabsorption in the loop of Henle and collecting duct. 3. The role of these peritubular factors in the natriuresis induced by maneuvers other than saline loading, including mannitol administration, hyperoncotic albumin, and acute hypertension.